1. Field of the Invention
The present invention relates to a semiconductor device such as IC and LSI, and particularly to a semiconductor device having lines (or interconnecting lines or wiring) which may reduce electromigration or/and stress migration. The present invention also relates to a process for producing a semiconductor device such as IC and LSI, particularly to a process for forming lines in a semiconductor device.
2. Related Background Art
In semiconductor devices, a deposited film (i.e., a film formed by sputtering or by vapor deposition) of aluminum (Al) is popularly used as a metal film for forming lines or wiring, because it has synthetically many advantages such as conductivity, ohmic contact property to silicon (Si), wire bonding property and processability. In a case where an aluminum wiring film is formed on a silicon substrate by use of a vapor deposition technique, the resultant wiring film generally has a polycrystal structure of aluminum grains. For example, as shown in a schematic view of FIG. 18, the above polycrystal structure comprises an aggregate of aluminum single crystal grains 1 wherein grain boundaries 2 are present among aluminum single crystal grains 1, and the wiring layer is imparted with conductivity on the basis of such a structure.
However, the deposited film as described above has a specific problem which is different from those encountered in wiring films generally used. More specifically, the deposited film has such a problem that disconnection or failure of line is liable to occur due to electromigration or/and stress migration. Accordingly, it is important to reduce the electromigration or/and stress migration in order to increase the lifetime of a semiconductor device.
The electromigration is a sort of diffusion phenomenon based on an interaction between metal atoms in a metal wiring film and electrons moving in the wiring film. More specifically, the electromigration is a phenomenon such that the metal atoms migrate in the same direction as that of the electron movement, and the resultant metal atom migration causes an atomic vacancy or void to be formed at a place from which the metal atoms have moved, or causes a hillock to be built up at a place where the metal atom accumulate, as shown in the enlarged view in FIG. 18. When the above void is formed as described above, a cross sectional area of the metal wiring film is decreased and consequently the current density in the metal wiring film is increased, whereby the temperature rises by Joule heat and the growth of void is further accelerated to invite disconnection of the line. On the other hand, the extension of the hillock as described above may result in a trouble of short circuit between proximate lines in a highly integrated circuit.
Further, although the mechanism of the stress migration phenomenon as described above is not completely clarified, it is considered that disconnection is caused on the basis of mechanical stress exerted onto the aluminum single crystal grains 1 shown in FIG. 18.
Hitherto, in order to improve an electromigration resistance of the wiring metal in a semiconductor integrated circuit, an Al-Cu alloy, in which Cu is added to Al, is used as a wiring material.
However, when the Al-Cu type alloy is used as a wiring material, there has been a problem that a sufficient resistance to electromigration cannot be obtained due to thermal hysteresis (or thermal history) in the wiring metal.
On the other hand, it is reported that the lifetime of the line can be improved by an isothermal storage treatment at 226.degree. C. for 24 hours after the formation of unpassivated line of an Al-2% Cu-1% Si alloy thin film (see "J. Appl. Phys. 72(5), p. 1837, Sep. 1, 1992"). However, in the case of the line of an alloy thin film having a relatively high Si content and containing a relatively large amount of Cu as described above, there is posed a serious problem of corrosion, as the line or interconnecting pattern is made finer (see "J. Vac. Sci. Technol. A, Vol. 8, No. 3, p. 2025, May/June 1990").
More specifically, it is known that rapid heating and rapid cooling leaves thermal hysteresis in a wiring metal, because a metal thin film deposition technique by sputtering is generally used for forming the wiring. Further, since an alloying step is conducted at about 400.degree. C. in a final step for producing a semiconductor integrated circuit, the thermal hysteresis is also imparted to the wiring metal due to rapid heating and rapid cooling. However, in the prior art, it is difficult to improve the resistance of the wiring metal to electromigration.